How to inspect userdata in lua

Question

I am using inspect.lua to inspect table to string.

But, if the value is a userdata, it returns just <userdata 1>

I really need to know what the userdata type is, what the userdata value is, it's very important for debuging, I don't want do it in any IDE, I just want something can help me debug by print staffs.

The only info that you can get is from `getmetatable(someuserdata)`. What exactly do you want to inspect? — Eric, Mar 02 '13 at 15:12
thanks a lot, there are something very useful, `print(inspect(getmetatable(someuserdata)))` — guilin 桂林, Mar 02 '13 at 15:17
where "someuserdata" is what, exactly?. And the return value of getmetatable is what exactly? The manual is useless. — CashCow, Jan 12 '15 at 15:51

score 16 · Answer 1 · answered Mar 18 '13 at 07:30

16

You cannot.

From the manual :

The type userdata is provided to allow arbitrary C data to be stored in Lua variables. A userdata value is a pointer to a block of raw memory. [...] Userdata has no predefined operations in Lua, except assignment and identity test.

As indicated by @Eric, the only thing you can do from Lua is inspect the metatable :

print(inspect(getmetatable(someuserdata)))

If you are using the C API, you should be able to register a custom function that prints whatever is held by the block.

answered Mar 18 '13 at 07:30

fouronnes

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and the identity test is? – CashCow Jan 12 '15 at 15:14
@CashCow: The identity test is ==. That is, the expression (someuserdata == someuserdata) returns true, in my admittedly limited test. Looks like ~= also works. – solublefish Mar 26 '16 at 00:27

score 0 · Answer 2 · answered Aug 28 '18 at 22:52

If the userdata metatable was created with luaL_newmetatable it contains an entry __name containing the userdatas type (since Lua version 5.3), see https://www.lua.org/manual/5.3/manual.html#luaL_newmetatable. Unfortunately this feature is still not widely used.

How to inspect userdata in lua

2 Answers2